1. Field of the Invention
This invention relates to vent valves, and in particular to a two-way vent valve specially adapted to equalize the pressure inside a liquid holding tank or reservoir to atmospheric pressure.
2. Discussion of the Prior Art
Hydraulic fluid is drawn from or added back to a hydraulic fluid reservoir, for example of a pump, according to the needs of the pump or hydraulic load to which the fluid is supplied. There is typically an air space in the reservoir, the volume of which varies with the amount of hydraulic fluid in the reservoir at any given time. When hydraulic fluid is drawn from the reservoir, the volume of the air space increases, and unless atmospheric pressure is admitted into the air space, a subatmospheric vacuum is drawn in the reservoir, which hampers the flow of hydraulic fluid out of the reservoir. If, on the other hand, fluid is added back into the reservoir, the volume of the air space is reduced, resulting in an increased, or superatmospheric, pressure, which acts on the fluid in the reservoir and is also undesirable.
The prior art has sought to provide a two-way vent valve in the filler cap of a liquid tank. However, many of these attempts have required many parts, parts which were difficult to make, or assemblies which resulted in inconsistent results, the pressure differential required to open the valve varying considerably. In addition, the purpose of these valves is to allow equalization of the pressure within the tank, without allowing egress of the liquid from the tank through the valve. In other words, the valve had to allow the passage of air at a certain low pressure differential, but not allow the passage of liquid.